In the production of disposable absorbent articles, pin aperturing is a widely used and inexpensive method for providing apertures in a web for purposes such as enhancing fluid transport, increasing void space, or improving the appearance of a product. Both single webs and composite webs can be pin apertured in an aperturing nip comprising a pin roll and a female roll. When an assembly of two or more layers are apertured simultaneously, the assembly is said to be coapertured. Aperturing and coaperturing can be desirable for many products, but they pose a number of challenges for high-speed manufacturing. Challenges arise in many cases due to in-plane deformation of the web or web assembly as it passes through an aperturing nip. As a pin first engages the web, the tip of the pin travels at a higher radial velocity than the perimeter of the opposing female roll, resulting in shear of the web near the tip of the pin. As the pin pushes the web down into the receiving recess of the female roll, more in-plane dislocation of the web occurs. The shear or dislocation experienced by portions of the web near the resulting apertures can reduce the clarity of the aperture. When the web has been previously embossed or otherwise textured, the clarity of the embossment or texture can suffer because of in-plane dislocation and shear of the web.
Examples of prior pin designs for aperturing webs are disclosed in U.S. Pat. No. 5,986,167, “Method of Distributing Liquid in Apertured Absorbent Pads,” issued Nov. 16, 1999 to Arteman and Myers; and U.S. Pat. No. 4,886,632, “Method of Perforating a Nonwoven Web and Use of the Web as a Cover for a Feminine Pad,” issued Dec. 12, 1989 to Van Iten et al., both of which are herein incorporated by reference. An exemplary pin design from the Van Iten patent is shown in FIG. 5, discussed hereafter
Particularly severe challenges are encountered in coaperturing web assemblies comprising two or more layers such as a high-bulk absorbent web of fluff disposed on a denser or substantially thinner nonwoven web or film. The dissimilar physical properties of the two or more layers can result in complex in-plane stresses and a variety of aperture defects or web deformation problems.
Designs for aperturing pins usually seek to minimize pin length because longer pins create a higher level of differential movement in the aperturing nip, cause more in-plane dislocation of the web, and are more likely to become jammed in the receiving recesses of the female roll. Nevertheless, conventional pin designs result in a variety of problems with aperture quality. It is believed that one problem with prior pin design has been the abbreviated length of the tapered section, which is often tapered at a high angle relative to the vertical axis, such as a 45 degree angle. The short span of the tapered section tends to result in greater web disruption and densification of the web near the aperture.
What is needed is an improved aperturing method wherein the in-plane dislocations are minimized, and specifically wherein the amount of material pushed into the receiving recesses of the female roll is minimized. Achieving this result requires consideration of the shape and length of the pins, the geometry of the receiving recesses of the female roll, and the properties of the web.